1. Field of the Invention
This invention relates in general to the fabrication of a flat panel display, and more particularly to a structure of a front plate for a plasma display panel (PDP) and a modified method of fabricating the front plate capable of reducing the number of photomasks required and improving the accuracy of the exposure and developing process.
2. Description of Related Art
Plasma display panels (PDPs) are generally classified into the DC type (or direct discharge type), in which the discharging electrodes are exposed in the discharge space, and the AC type (or indirect discharge type), in which the discharging electrodes are covered with a dielectric layer. AC type PDPs are further classified into two types: one is a facing surfaces charging type in which the discharging electrodes are provided onto two substrates of back and front sides respectively; the other is a surface discharge type in which the discharging electrodes are provided onto only one of two substrates of back and front sides.
The AC type PDP is driven by a voltage application method such as the refreshing method, the matrix addressing method, the self-shifting method, etc. FIG. 1, for example, shows a surface discharge AC type PDP with a matrix addressing method which comprises a front plate 10 and a back plate 11 facing and parallel to each other, and a discharge gas space 18 defined by these substrates and barrier ribs of an insulating material (not shown). The barrier ribs partition pixel cells to prevent adjacent cells from leaking ultraviolet rays produced by the electrical discharge.
In the front plate 10, a plurality of pairs of sustaining electrodes are formed parallel to each other on the inside as row electrodes per one pixel cell. Each sustaining electrode comprises a transparent electrode 12 and a metal electrode 12a with a narrower width thereon. As illustrated in FIG. 1, a gap G is shown between each pair of the transparent electrodes. A dielectric layer 13 is uniformly formed on and over the sustaining electrodes. A protective layer 14, such as a MgO layer, is then formed on the dielectric layer 13.
In the back plate 11, address electrodes 15 are formed parallel to each other on the inside as column electrodes in such a manner that each address electrode crosses a sustaining electrode. Fluorescent layers 16 are formed on the internal surface of the back plate 11 so as to correspond to unit pixel cells, respectively. The front plate 10 and the back plate 11 are assembled after being aligned in a way that each address electrode and each sustaining electrode crossover apart from each other at an intersection space 18 for a discharge-oriented emission corresponding to one pixel cell, and then the discharge space 18 is filled with a rare gas mixture. In this way, a surface discharge type PDP is manufactured.
This PDP is operated as follows: when a predetermined voltage is applied across each pair of the address electrodes and the sustaining electrodes embedded in the dielectric layer 13, a discharging region appears above the dielectric layer 13 at the crossover point of each pair of electrodes in the gaseous space 18. Ultraviolet rays emitted from the discharging region stimulate the fluorescent layer 16 to emit light radiating through the front plate 10 as an emission region. This discharged emission is maintained by a sustaining voltage applied between the sustaining electrodes, but canceled by an erase pulse applied between the address electrodes.
The PDP has been considered the most suitable flat device for a large size displays (i.e., those exceeding over 20 inches) because high-speed display is possible and a large size panel can easily be made. In the conventional fabrication of a front plate of the PDP, three photomasks are required to perform the necessary exposure and developing processes. These include a photomask for transparent electrodes, a photomask for metal electrodes, and a photomask for black stripes. When performing the exposure process, a charge-coupled device (CCD) is used to detect an alignment mark on the substrate, and then a step motor is used to position the substrate or the photomask accordingly to ensure the highest alignment accuracy. As the manufacture steps proceeding, several layers of different materials are successively formed on the substrate; the alignment mark should be always transferred onto the upper most layer.
However, if the alignment mark is transferred to a layer with high transparency, such as a transparent electrode, the normal auto-alignment exposure process cannot be achieved since the stepper is unable to detect the alignment mark. Therefore, a manual exposure process should be performed as an alternative, which not only increases the process time in a manner unfavorable to manufacturing efficiency, but also reduces the exposure accuracy and thus influences the uniformity of the product device. To achieve a good understanding the above-mentioned problem, please now refer to FIGS. 2A to 2J showing the processing steps of fabricating a front plate for a plasma display panel by a prior art method.
First, as shown in FIG. 2A, a substrate 20 such as a glass plate is provided. A transparent conductive layer 21, such as an Indium tin oxide (ITO) layer, is formed overlying the surface of the substrate 20. Next, referring to FIG. 2B, a negative-type photoresist layer 22 is coated on the transparent conductive layer 21. An exposure and developing process is then performed by using a first photomask 23 to define a photoresist pattern 22a that covers portions of the transparent conductive layer 21 for forming transparent electrodes, as can be seen in FIG. 2C. Then, the transparent conductive layer 21 is etched using the photoresist pattern 22a as a mask to form a plurality of pairs of transparent electrodes 21a parallel to each other. After removing the photoresist pattern 22a by using an appropriate solvent or dry etching, the resulting structure is shown in FIG. 2D.
Then, as can be seen in FIG. 2E, a laminated metal layer 24 is formed on the transparent electrodes 21a and the substrate 20. For example, the laminated metal layer 24 includes a chromium layer 241, a copper layer 242, and another chromium layer 243 (Cr/Cu/Cr). Referring to FIG. 2F, a positive photoresist layer 25 is coated on the laminated metal layer 24. A second exposure and developing process is then performed by using a second photomask 26 to define a photoresist pattern 25a that covers portions of the laminated metal layer 24 for forming metal electrodes, as can be seen in FIG. 2G. Then, the laminated metal layer 24 is etched using the photoresist pattern 25a as a mask to form a plurality of pairs of metal electrodes 24a on the corresponding transparent electrodes 21a. After removing the photoresist pattern 25a by using an appropriate solvent or dry etching, the resulting structure is shown in FIG. 2H.
Referring to FIG. 21, a light-shielding layer 27, such as the PbO--Ba.sub.2 O.sub.3 --SiO.sub.2 series materials layer, is formed overlying the exposed surfaces of the transparent electrodes 21a, the metal electrodes 24a and the substrate 20. Using a third photomask 28, a third exposure and developing process is then performed to the light-shielding layer 27 to form a plurality of so-called black stripes (also called black belts) 28 on the gaps between each pair of the transparent electrodes 21a. Thereafter, a dielectric layer and a passivation layer (not shown) are successively formed to complete the fabrication of the front plate for a PDP.
In the above conventional fabricating process, the first photomask 23 is auto-aligned to a desired position either by a side-by-side alignment or by using a predefined alignment mark. This can be done easily within 10 seconds using today's manufacturing platen. However, this auto-alignment scheme cannot be achieved when the second photomask is used, because the alignment mark is transferred onto the transparent conductive layer 21. Since the exposure platen's detector is unable to detect the transparent alignment mark automatically, a manual alignment process is performed instead. This results in the increase of the processing time and the reduction of the exposure accuracy. After that, the alignment mark is transferred onto the laminated metal layer 24. The auto-alignment scheme again can be applied to the third photomask to execute another exposure process.
Hence, a modified method of fabricating a front plate for a PDP has been disclosed, which is able to perform all of the exposure and developing processes using auto-alignment changing the sequence of forming the transparent electrodes and the metal electrodes. A detailed explanation of this prior art modified method is described with reference to accompanying FIGS. 3A to 3I. First, as shown in FIG. 3A, a substrate 20 such as a glass plate is provided. Next, a transparent conductive layer 21, such as an Indium tin oxide (ITO) layer; and a laminated metal layer 24, for example, a stacked structure of chromium layer 241/copper layer 242/chromium layer 243 (Cr/Cu/Cr) are formed successively overlying the substrate 20.
Referring to FIG. 3B, a positive-type photoresist layer 25 is coated on the laminated metal layer 24. An exposure and developing process is then performed by using the second photomask 26 to define a photoresist pattern 25a that covers portions of the laminated metal layer 24 for forming metal electrodes, as can be seen in FIG. 3C. Next, the laminated metal layer 24 is etched using the photoresist pattern 25a as a mask to form a plurality of pairs of metal electrodes 24a, as shown in FIG. 3D. After that, the photoresist pattern 25a is removed by using an appropriate solvent or dry etching.
Subsequently, referring now to FIG. 3E, a negative-type photoresist layer 29 is coated on the transparent conductive layer 21 and the metal electrodes 24a. Another exposure and developing process is then performed by using the first photomask 23 to define a photoresist pattern 29a that covers portions of the transparent conductive layer 21 for forming transparent electrodes, as can be seen in FIG. 3F. The transparent conductive layer 21 is etched using the photoresist pattern 29a as a mask to form a plurality of pairs of transparent electrodes 21a parallel to each other. After removing the photoresist pattern 29a by using an appropriate solvent or dry etching, the resulting structure is shown in FIG. 3G.
Referring to FIG. 3H, a light-shielding layer 27, such as the PbO--Ba.sub.2 O.sub.3 --SiO.sub.2 series materials layer, is formed overlying the exposed surfaces of the transparent electrodes 21a, the metal electrodes 24a and the substrate 20. Using the third photomask 28, a third exposure and developing process is then performed to the light-shielding layer 27 to form a plurality of black stripes 27a on the gaps between each pair of transparent electrodes 21a, as can be seen in FIG. 31. Thereafter, a dielectric layer and a passivation layer (not shown) are successively formed to complete the fabrication of the front plate for a PDP.
Compared to the conventional method described in FIGS. 2A to 2J, this prior art modified method is able to perform the entire exposure process in an auto-alignment scheme. First, the second photomask 26 is used to define the photoresist pattern 25a with an auto-aligned exposure and developing process. Next, an auto-alignment can be achieved in the exposure process using the first photomask 23 since the alignment mark transferred onto the laminated metal layer 24 is easily detected by the exposure platen's detector. Finally, the alignment mark in the laminated metal layer 24 is utilized to execute another auto-aligned exposure process for forming the black stripes 27a in the exposure process that uses the third photomask 28. With this entirely auto-aligned scheme, the exposure accuracy and the production efficiency can be improved. However, along with the continuous development of PDP manufacture technology, there remains a need to make further modifications to the processing steps to achieve better production efficiency.